Such electric circuits are for example used in different types of converters, rectifiers and inverters, in which high voltages are to be handled. The use of such an electric circuit in a converter in a station for converting direct voltage into alternating voltage may be mentioned as a non-limitative example.
The property in common to all these applications is that the voltages to be held in the blocking state of the single switch are too high to be held by one single switching device, so that a plurality of such devices has to be connected in series for distributing the voltage to be held by the switch thereamong. The voltage to be held may for instance be 30-500 kV, and each switching device may for example alone only hold 1-10 kV. The problem is that it is not possible to manufacture switching devices so that they have exactly the same properties, i.e. reacts exactly the same in every single instant. This means that an over-voltage, i.e. a voltage being higher than the average voltage to be held by the switching devices, may easily occur for an individual switching device during switching due to differences in turn-on delay times, turn-off delay times, tail currents and reverse recovery charges of opposite diodes in a phase leg of a voltage source converter in the case that the single switch forms a current valve of such a converter. It is essential to protect the different switching devices against such overvoltages, since they may destroy the device and possibly result in a drop out of a larger unit, such as a converter station with an interruption of the operation thereof being very costly as a consequence.
It is not a realistic solution to this problem to arrange that many switching devices in series in such a switch without additional surrounding components that the over-voltages will always be within an acceptable limit, but it is a desire to keep the number of switching devices in such a single switch as low as possible for saving costs for the equipment for controlling the switching devices, cooling them and so on. It has therefore been necessary to design the means for controlling the switching devices so that over-voltages occurring will be kept within an acceptable area with respect to magnitude and duration, so that they will not be harmful to the switching devices. This normally means that each individual switching device has to be provided with a separate control member and the control of the single switch has been made very complex and intelligent gate drive circuitry limiting the voltage-time gradient has been utilized in order to control the quotient for each individual switching device. This means that the cost of those electric circuits already known is comparatively high. Another problem always existing in such electric circuits is that the power losses in different components are higher than desired.